The present invention relates to a semiconductor photodetector, and more particularly to a semiconductor photodetector exhibiting a high speed response and having a high external quantum efficiency.
FIG. 1 is a fragmentary cross sectional elevation view illustrative of a conventional semiconductor photodetector. The conventional semiconductor photodetector is provided over a semi-insulating InP substrate 20. An n+-InGaAsP layer 21 having a thickness of 0.2 micrometers is selectively formed on a first region of an upper surface of the semi-insulating InP substrate 20. A first polyimide insulating layer 27a is also selectively formed on a second region of the upper surface of the semi-insulating InP substrate 20. A second polyimide insulating layer 27b is selectively formed on a first region of an upper surface of the n+-InGaAsP layer 21. The second polyimide insulating layer 27b has a window through which the upper surface of the n+-InGaAsP layer 21 is shown. An n-contact 28 of AuGaNi is formed in the window and on the upper surface of the n+-InGaAsP layer 21, so that the n-contact 28 is surrounded by the second polyimide insulating layer 27b. A multi-layered structure is provided selectively provided on a second region of the upper surface of the n+-InGaAsP layer 21, so that the multi-layered structure is surrounded by the first and second polyimide insulating layers 27a and 27b. The multi-layered structure comprises the following five layers 22, 23, 24, 25 and 26. An undoped InGaAs optical absorption layer 22 having a thickness of 0.4 micrometers is selectively provided on the second region of the upper surface of the n+-InGaAsP layer 21. A p+-InGaAs optical absorption layer 23 having a thickness of 0.2 micrometers is laminated on the undoped InGaAs optical absorption layer 22. A p+-InGaAsP layer 24 having a thickness of 0.2 micrometers is laminated on the p+-InGaAs optical absorption layer 23. A p+-InP layer 25 having a thickness of 0.5 micrometers is laminated on the p+-InGaAsP layer 24. A p+-InGaAsP layer 26 having a thickness of 0.2 micrometers is laminated on the p+-InP layer 25. An upper surface of the p+-InGaAsP layer 26 is leveled to the top surfaces of the first and second polyimide insulating layers 27a and 27b to form a palatalized surface. A p-contact 29 of AuZnNi is provided on the palatalized surface, wherein the p-contact 29 is in contact with the p+-InGaAsP layer 26. The n-contact 28 and the p-contact 29 are electrically connected to each other through the n+-InGaAsP layer 21, the undoped InGaAs optical absorption layer 22, the p+-InGaAs optical absorption layer 23, the p+-InGaAsP layer 24, the p+-InP layer 25 and the p+-InGaAsP layer 26. The optical waveguide comprises the n+-InGaAsP layer 21, the undoped InGaAs optical absorption layer 22, the p+-InGaAs optical absorption layer 23, the p+-InGaAsP layer 24, the p+-InP layer 25 and the p+-InGaAsP layer 26. The n-contact 28 is bonded through a bonding wire to a pad formed on the top surface of the second polyimide insulating film 27b. The above undoped InGaAs optical absorption layer 22 further comprises three lamination layers of a pxe2x88x92-InGaAs layer, an i-InGaAs and a pxe2x88x92-InGaAs layer.
In order to realize the high speed response of the conventional semiconductor photodetector, a total thickness of the undoped InGaAs optical absorption layer 22 and the p+-InGaAs optical absorption layer 23 is thin, for example, 0.6 micrometers which is less than 1 micrometer. The reduction in the total thickness of the undoped InGaAs optical absorption layer 22 and the p+-InGaAs optical absorption layer 23 shortens a carrier traveling time to improve the high speed response of the conventional semiconductor photodetector. The reduction in the total thickness of the undoped InGaAs optical absorption layer 22 and the p+-InGaAs optical absorption layer 23, however, raises a problem with a reduction in coupling efficiency to an incident light from an optical fiber. An optical absorption region of the above conventional semiconductor photodetector comprises the n+-InGaAsP layer 21, the undoped InGaAs optical absorption layer 22, the p+-InGaAs optical absorption layer 23 and the p+-InGaAsP layer 24. A total thickness of the n+-InGaAsP layer 21, the undoped InGaAs optical absorption layer 22, the p+-InGaAs optical absorption layer 23 and the p+-InGaAsP layer 24 is 1 micrometer. Namely, the thickness of the optical absorption region of the above conventional semiconductor photodetector is 1 micrometer. A spot size of the incident light from the optical fiber is, however, about 9 micrometers. namely, the thickness of the optical absorption region of the above conventional semiconductor photodetector is much smaller than the spot size of the incident light from the optical fiber, for which reason an external quantum efficiency is low.
In the above circumstances, it had been required to develop a novel semiconductor photodetector free from the above problems.
Accordingly, it is an object of the present invention to provide a novel semiconductor photodetector free from the above problems.
It is a further object of the present invention to provide a novel semiconductor photodetector exhibiting a high speed response and having a high external quantum efficiency.
It is a still further object of the present invention to provide a novel semiconductor photodetector exhibiting a high speed response at about 20 GHz and having a high external quantum efficiency of not less than 90%.
It is yet a further object of the present invention to provide a novel semiconductor photodetector operable at a low voltage of not more than 1 volt and having a high reliability and a stable dark current characteristic.
The present invention provides an optical waveguide structure comprising plural periods of a multi-layered structure which comprises an InGaAs optical absorption layer of a first conductivity type, a pair of first and second InGaAsP cladding layers of the first conductivity type sandwiching the InGaAs optical absorption layer, and a pair of a first InP layer of the first conductivity type and a second InP layer of a second conductivity type, and the first and second InP layers sandwiching the first and second InGaAsP cladding layers.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.